1. Field of the Invention
This invention relates to a bi-directional optical wireless communication apparatus and a method of bi-directional optical wireless communication.
2. Description of the Prior Art
A bi-directional optical wireless communication apparatus communicating with another bi-directional optical wireless communication apparatus is known.
A prior art bi-directional optical communication apparatus is disclosed in Japanese patent application No. 6-209080 by the same inventor of this invention.
In this prior art bi-directional optical communication apparatus transmits a first broad band signal and a first narrow band signal superimposed each other and receives the reflected first broad band signal and the reflected first narrow band signal in addition to a second broad band signal, having the same broad band, and a second narrow band signal having a different narrow band transmitted from another bi-directional optical apparatus and cancels the reflected first broad band signal to detect the second broad band signal in accordance with a control signal indicative of excess or insufficiency of a cancelling amount which is generated through phase comparing the first narrow band signal with the reflected first narrow band signal.
FIG. 5 is a block diagram of this prior art bi-directional optical communication apparatus. FIG. 6 is a graphical drawing of a spectrum representing bands of optical signals used in this prior art bi-directional optical communication apparatus.
A video input signal is modulated by a video FM modulation circuit 1 and the sound input signal is modulated by a sound FM modulation circuit 2. Both modulation signals are added by an adding circuit 3. An output of the adding circuit 3 is transmitted by an emitting element 5 such as an LED through a driver 4 as an optical output signal.
On the other hand, the receiving element 6 receives optical signals and generates a received signal in cooperation with a receiving circuit 7. The received signal is amplified by an amplifier 9 via an adding circuit 8 and applied to a band pass filter (BPF) 10 for a video signal, a band pass filter 11 for a sound signal, and a band pass filter 12 for a sound signal transmitted by this bi-directional communication apparatus. The band pass filter 10 transmits a frequency band of the video signal and the band pass filter 11 transmits a transmitted signal having a sound frequency band. Outputs of the band pass filters 10 and 11 are supplied to modulation circuits 13 and 14 for the video signal and the sound signal respectively. The modulation circuit 13 and 14 output a video output signal and a sound output signal respectively.
As shown in FIG. 6, the transmitted and received video signals have the same band from 6-20 MHz because the emitting element 5 such as the LED has a frequency response up to 20 to 30 MHz, the video signal has a broad band at least 0 to 5 MHz which varies with an modulation index, and there is interference light components up to 1 MHz. On the other hand, the sound signals are transmitted through different bands in respective directions. Then, receiving the video signal requires cancellation of the video signal transmitted by this bi-directional communication apparatus which is reflected by a reflective object. The adder 8 cancels the reflected video and sound signals which are transmitted by this bi-directional communication apparatus in the received signal by the receiving element 6 with the video signal and sound signal from the adder 3. However, an excessive cancellation provides an inverted component of the video signal and the sound signal from the adder 3 at the output of the adder 8. On the other hand, an insufficient cancellation leaves the reflected video and sound signals at the output of the adder 8. A necessary degree of the cancellation is detected by phase comparing the sound modulation signal from the modulator 2 with an output of the band-pass filter 12 extracting the reflected sound signal, by a phase comparator 17. Therefore, an output of the phase comparator 17 represents the excessive and insufficient condition of the cancellation. A low pass filter 18 smooths an output of the phase comparator 17. Then, a voltage controlled amplifier adjusts the degree of cancellation. When the amplitude of the voltage controlled amplifier 15 equals to that of the received reflected optical signal, the reflected optical signal including the video signal and the sound signal transmitted from this bi-directional communication apparatus is preferably suppressed. The output of the voltage controlled amplifier 15 is phase compensated by a phase compensation circuit 16 and supplied to a minus input of the adder 8.